Vertical Surface-emitting Laser Research Articles

Maximizing capacity, flexibility and efficiency in G-PON networks using VCSEL-based OOK and 2/4-PAM formats

ABSTRACTThis work experimentally demonstrates the potential of multi-level pulse amplitude modulation with direct detection to maximize carrier spectral efficiency and double the Gigabit passive optical networks (G-PON) network data rate. Three scenarios have experimentally been exploited. First, a 1310 nm vertical surface-emitting laser (VCSEL) has directly been modulated with a 10 Gbps OOK data. A receiver sensitivity of −19.11 dBm is attained, and a successful error free transmission over 22 km SMF fibre achieved, with a transmission penalty of 0.46 dB. To maximize carrier spectral efficiency, 2/4 PAM modulation formats are adopted respectively. A receiver sensitivity of −14.64 and −11.63 dBm is attained for 2-PAM and 4-PAM formats respectively. However, a 3.21 km fibre transmission introduces a penalty of 0.64 and 3.30 dB for 2-PAM and 4-PAM formarts respectively. 2/4-PAM modulation formats significantly increase the aggregated data rate at different ONUs within the G-PON without expensive optics investment, though at the cost of reduced transmission reach due to the high bitrate attained. We further demonstrate the design of a software defined digital signal processing assisted receiver to recover the 2/4 PAM transmitted signal without employing costly receiver hardware.

Anisotropy in the wet thermal oxidation of AlGaAs: influence of process parameters

The crystallographic anisotropy of the lateral selective thermal oxidation of AlGaAs alloys is experimentally studied. The anisotropic behavior of this oxidation process, used primarily for building a lateral confinement in vertical surface emitting lasers (VCSEL), is quantified by varying different process parameters and the geometrical shapes of laterally oxidized mesa structures. This experimental study aims to have a better control of the oxide aperture shape used in oxide-confined photonics devices.

Optical Characterizations of VCSEL for Emission at 850 nm with Al Oxide Confinement Layers

In-plane micro-photoluminescence (μ-PL) and micro-reflectivity measurements have been performed at room temperature by optical excitation perpendicular to the surface of two different structures: a complete vertical surface-emitting laser (VCSEL) structure and a VCSEL without the upper p-type distributed Bragg reflector (P-DBR). The two structures were both laterally oxidized and measurements were made on the top of oxidized and unoxidized regions. We show that, since the photoluminescence (PL) spectra consist of the cumulative effect of InGaAs/AlGaAs multi-quantum wells (MQWs) luminescence and interferences in the DBR, the presence or not of the P-DBR and oxide layers can significantly modify the spectrum. μ-PL mapping performed on full VCSEL structures clearly shows oxidized and unoxidized regions that are not resolved with visible light optical microscopy. Finally, preliminary measurements of the degree of polarization (DOP) of the PL have been made on a complete VCSEL structure before and after an oxidation process. We obtain an image of DOP measured by polarization-resolved μ-PL. These measurements allow us to evaluate the main components of strain.

Efficient Heat Dissipation of Uncooled 400-Gbps (16\xd725-Gbps) Optical Transceiver Employing Multimode VCSEL and PD Arrays

An effective heat dissipation of uncooled 400-Gbps (16×25-Gbps) form-factor pluggable (CDFP) optical transceiver module employing chip-on-board multimode 25-Gbps vertical-surface-emitting-laser (VCSEL) and 25-Gbps photodiode (PD) arrays mounted on a brass metal core embedded within a printed circuit board (PCB) is proposed and demonstrated. This new scheme of the hollow PCB filling with thermally-dissipated brass metal core was simulated and used for high temperature and long term stability operation of the proposed 400-Gbps CDFP transceiver. During one-hour testing, a red-shift of central wavelength by 0.4-nm corresponding temperature increment of 6.7 °C was observed with the brass core assisted cooler module. Such a temperature change was significantly lower than that of 28.3 °C for the optical transceiver driven with conventional circuit board. After 100-m distance transmission over a multimode fiber (OM4), the 400-Gbps CDFP transceiver exhibited dispersion penalty of 2.6-dB, power budget of ≧ 3-dB, link loss of ≦ 0.63-dB, mask margin of 20%, and bit error rate (BER) of <10−12 with maintained stability more than one hour. The developed 400-Gbps CDFP transceiver module employing low-power consumption VCSEL and PD arrays, effective coupling lens arrays, and well thermal-dissipation brass metal core is suitable for use in the low-cost and high-performance data center applications.

(Invited) New Directions in GaN Photonics Enabled by Electrochemical Processes

We have developed a novel conductivity based selective electrochemical etching to introduce nanometer sized pores into GaN. The nanoporous (NP) GaN can be considered as a new form of GaN with an unprecedented tunability in optical index. The advantages of NP-GaN for both edge-emitting laser diodes and vertical surface-emitting laser diodes (VCSEL) are subsequently exhibited.

Performance Analysis of Vertical and Horizontal Transmitter Array Modules Using Short- and Long-Wavelength VCSELS for Optical Interconnects

For short- and long-wavelength vertical surface-emitting lasers (VCSELs), performances of optical transmitter array modules with vertically and horizontally packaged structures are analyzed for application to on-board optical interconnection. 850 and 1310 nm wavelengths are selected for the short- and long-wavelength light sources. For each wavelength, top- and bottom-emitting VCSELs are used to compare the performances of four different module structures: bottom-emitting types of 850 and 1310 nm for the vertical modules, and top-emitting types of 850 and 1310 nm for the horizontal modules. The horizontal transmitter modules are packaged using the conventional wire-bonding technology for the interconnect between VCSEL and driver chips while the vertical modules are packaged using the flip-chip-bonding technology. Signal crosstalk is investigated using 1 $\,\times\,$ 4 VCSEL array chips, which are driven using the same Si-CMOS circuits designed for 5 Gb/s operation. The vertical modules show lower crosstalk than the horizontal modules. When the gap distance between the VCSEL aperture and the waveguide input port increases up to 500 $\mu{\rm m}$ for each module, the crosstalk increases in a range ${-}{58}$ to ${-}{\rm 37}~{\rm dB}$ . The 3-dB bandwidth performances are sensitively degraded in a range 4.6–2.0 GHz when the gap distance increases. In the crosstalk, the 1310-nm vertical module shows the best performance, while in the 3-dB bandwidth, the 850-nm horizontal module shows the best performance. The four modules shows clear eye openings at 2.5 Gb/s with bit error rate ${ and can be used for on-board optical interconnections.

An Antenna Measurement System Based on Optical Feeding

A radiation measurement system by using optical feeding is proposed. The system replaces conventional electrical feeding to antennas by the optical feeding which is composed of an electrical/optical (E/O) converter, a graded-index (GI) optical fiber, and an optical/electrical (O/E) converter. The GI fiber is used so as the O/E converter becomes very compact by using a simple means of coupling between the fiber and the photo-diode in the converter. A vertical surface emitting laser (VCSEL) is used in the E/O converter to make the system available till 6 GHz. This combination also makes the system cost-effective. The validity as well as the advantage of the system is demonstrated by measuring an ultra-wideband (UWB) antenna both by the optical and electrical feeding systems and comparing with a calculated result. Ripples in radiation pattern due to the electrical feeding are successfully suppressed by the optical feeding. For example, in a radiation measurement on the azimuth plane at 3 GHz, ripple amplitude of 1.0 dB that appeared in the electrical feeding is reduced to 0.3 dB. In addition, a circularly polarized (CP) antenna is successfully measured by the proposed system to show that the system is available not only for amplitude but also phase measurements.

On the condensation of polaritons

Bose–Einstein condensates (BECs), the “fifth state of matter,” have in fact been discovered by chance both in liquid helium and in superconductors. Since these early encounters, BECs have been sought for, both in atom vapors and in solids. Here we report on the case of exciton polaritons. We discuss the experimental observation of macroscopically occupied polariton states and their possible attribution to a BEC. We also discuss the possible relation between a polariton condensate, a polariton laser, and a vertical surface-emitting laser (VCSEL). Vortices, superfluidity, Bogoliubov dispersion, half-vortices, and Josephson oscillations are then briefly summarized merrily for sake of discussion of the distinction between a polariton BEC and a VCSEL.

90$^{\circ}$ Light Beam Deflection Technology for Single-Mode Optical Interconnection Systems

In this letter, we demonstrate an extremely low loss (<0.06 dB) 1-mm bending radius 90deg light deflection by using a 12-single-mode-fiber ribbon for optical interconnection systems. This technology is easy to be adopted for optical surface mount assemblies that need light beam deflection between the vertical surface-emitting lasers or surface-receiving photodiodes and optical paths parallel to an electric circuit board. We have chosen single-mode fiber (SMF) for the 90deg-bent fiber expecting the single-mode transmission will be applied for high-performance optical interconnection systems because of its wide bandwidth and stability in transmission characteristics. In order to minimize the bending loss and accomplish a small bending radius, we have developed a new optical fiber having higher refractive index of the core (high delta n fiber) than that of conventional fiber and 80-mum-cladding 125-mum spacing 12-fiber ribbon for dense packaging and easy handling in narrow spaces. It is bent at a radius of 1 mm by heating process with a ceramic heater.

10 Gbit∕s 2 km photonic crystal fibre transmission with 850 nm directly modulated singlemode VCSEL

Described is a 10 Gbit/s transmission at 850 nm realised using a 2 km-long singlemode photonic crystal fibre and a directly modulated singlemode vertical surface-emitting laser (VCSEL). Power penalty-free transmission was successfully achieved without an optical amplifier or a modulator.

Fabrication of a 2.5 Gbps × 4 channel optical micro-module for O-PCB application

We report on the fabrication of a polymer-based 2.5 Gbps × 4 channel optical interconnecting micro-module for optical printed circuit board (O-PCB) application. An optical waveguide array is used for optical transmission from vertical surface emitting laser (VCSEL) array to photodiode (PD) array and the built-in 45° waveguide mirrors are used for vertical coupling. The optical waveguide array and the 45° mirrors are fabricated by UV imprint process in one-step. We fabricate microlensed VCSELs by micro-inkjetting method, which reduced radiation angle of VCSEL from 18° to 15° for better light coupling. We use solder ball array and pin array for alignment between O-PCB and the electrical sub-boards with alignment mismatch below 10 μm in x, y and z axis. The fabricated optical interconnection module transmits data at the rate of 2.5 Gbps per channel.

High speed performance of 850 nm silicon-implanted AlGaAs/GaAs vertical cavity emitting lasers

In this paper, we report a high speed performance of silicon-implanted vertical surface emitting lasers (VCSELs) with the aperture size 6 × 6 μm 2. These VCSELs exhibit kink-free current–light output with threshold currents ∼1.14 mA, and the slope efficiencies ∼0.5 W/A. The eye diagram of VCSEL operating at 10 Gb/s with 6 mA bias and 6 dB extinction ratio shows very clean eye. The rise time is 29 ps and fall time is 41 ps with jitter (p–p)=21 ps. We have accumulated life test data up to 1000 h at 70 °C/10 mA.

Performance of 850 nm AlGaAs/GaAs implanted VCSELs utilizing silicon implantation induced disordering

In this paper, we report a novel implanted vertical surface emitting lasers (VCSELs) utilizing silicon implantation induced disordering. The VCSELs exhibit kink-free current–light output performance with threshold currents ∼2.4 mA, and the slope efficiencies ∼0.45 W/A. The threshold current change with temperature is minimal and the slope efficiency drops less than ∼30% when the substrate temperature is raised to 90 °C. The eye diagram of VCSEL operating at 2.125 Gb/s with 7 mA bias and 10 dB extinction ratio shows very clean eye with jitter less than 30 ps. We have accumulated life test data up to 5000 h at 100 °C/20 mA with exceptional reliability and the WHTOL (high temperature and high humidity 85 °C/85 operating lifetime) biased at 8 mA has passed over 2000 h.

Electroluminescence studies of stacked self-assembled InAs/GaAs-quantum dots embedded in a Bragg resonator

In regard to achieve a vertical surface-emitting laser with quantum dots as active medium, we have realized a microcavity consisting of two distributed Bragg reflectors and three layers of self-assembled InAs quantum dots and a similar sample without DBRs for comparison. The electroluminescence of the sample without cavity is rather broad due to size fluctuations of the islands, whereas the electroluminescence of the cavity sample is dominated by a narrow (FWHM 0.38 meV) and intense line at the resonance energy of the cavity. Several transverse modes are observed due to the wave-guiding effect of the etched mesa and identified by a simple model.

High performance microcavity resonator devices grown by atmospheric pressure MOVPE

Atmospheric pressure MOVPE has been used to investigate two GaAs based microcavity resonator devices, the “vertical surface emitting laser (VSEL) and the asymmetric Fabry-Pérot modulator”. The building blocks for both devices include (Al)GaAs/AlAs distributed Bragg reflectors (DBRs), GaAs/AlGaAs MQWs and InGaAs/GaAs pseudomorphic QWs. The effect of precursor impurities on device operation is discussed together with the assessment techniques of optical reflection, X-ray diffraction, dissolution profiling and photovoltage spectroscopy. AFPM arrays with 21 isolated devices showed approximately 60% reflection change for 9.5 V bias. 50 μm diameter VSEL devices have been fabricated using DBRs grown with and without intermediate layers. The inclusion of 200 Å Al 0.3Ga 0.7As at each interface of the DBR dramatically reduces the operating voltage of the laser.